LISBON (AFP) -- Portugal's prime minister has come out in support of holding a referendum on a much-contested future EU constitution, whose proposed draft Portugal and other small states feel needs serious revising.

Prime Minister Jose Manuel Durao Barroso told a national meeting of his Social Democratic Party late Tuesday he felt a public referendum should be held on any European Union constitution at the same time as the next elections for the European Parliament in June 2004.

Portugal, already an EU state, is among 15 current and future members who have criticized a draft text put to a Rome summit last weekend, which they feel would concentrate power in the hands of bigger member-states, notably Germany, France and Britain.

Barroso said he had no intention of ratifying any EU constitution "against the will of the Portuguese people". Any treaty needed to be subject to political debate and "legitimized by the people" before it takes effect, he said.

EU heavyweights had hoped negotiations on the constitution would be completed by December, but sharp divisions at the Rome summit made this prospect unlikely.

Portuguese President Jorge Sampaio, meanwhile, invited five European counterparts to an informal summit to "promote dialogue and debate on the ways and future of Europe", a statement from his office said Wednesday.

The invitation was sent to the heads of state of Germany -- a forceful supporter of the draft constitution and the largest contributor to EU subsidy programs -- and four smaller or medium states who, like Portugal, want changes in the treaty: Finland, Hungary, Latvia and Poland.

The summit is scheduled for October 18 and 19 in Arraiolos, a small town near Evora in the south of the country.

The proposed constitution would enforce a total shake-up of the EU's creaking institutional architecture, as it expands from 15 member countries to 25 next May -- and beyond in the next few years.

The 15 smaller states notably oppose replacing the EU's rotating presidency with a more permanent president and reducing the number of EU commissioners from 20 to 15, just as membership of the Union is set to expand to 25.

Public referenda -- expected to be held in several countries -- could make the road rockier, by highlighting the divisions among big and small states.